This invention relates generally to a semiconductor package, and more particularly to the impedance of conductors in a semiconductor package.
In order to maintain signal integrity, the design of high speed integrated circuit (IC) devices requires specific characteristic impedance of conductors which interconnect an IC chip to a printed wiring board or the next level of interconnection. For example, clock circuit drivers of many high frequency circuits are designed for 50 ohm matched impedance, and this in turn necessitates a specific geometric conductor design, and a predetermined output location for the conductor system in a package or substrate to match the circuit design. Moreover, the conductor system within a substrate or package used for both single ended and differential signal transmission has different impedance levels for each mode, and thus a specific conductor design for individual devices.
Arrays of conductors provide transmission lines in semiconductor packages or substrates; these arrays or systems include signal layer(s), and a ground conductor(s) which may be within the substrate, the printed wiring board or the semiconductor chip itself. The conductive layers are interspaced with dielectric materials. Characteristic impedance of the conductors is a function of conductor spacing, dielectric thickness, conductor width, the electrical properties of the materials, and the velocity of signal propagation on the line.
Differential impedance between a pair of conductors is either odd mode or even mode depending upon the direction of signal propagation, and for some circuits the need for both modes exists within the same device. This is particularly true of higher frequency and microwave devices designed for even mode impedance, often at 100 ohms.
The ongoing proliferation of high speed circuits with specific conductor requirements places a difficult and costly demand for as many package or substrate designs. A need exists for more flexible conductor systems which accommodate a number of different circuit design requirements.
The primary object of the invention is to provide a multiple use conductor system for interconnecting an integrated circuit chip to an external circuit wherein the impedance level of the conductors serving as transmission lines can be arbitrarily selected.
It is an object of the invention to provide a semiconductor package having conductors wherein the impedance can be selected for individual leads or lead pairs.
It is an object of the invention to provide a semiconductor package wherein the characteristic impedance of the conductors can be selected for single ended or differential pairs of transmission lines, as well as for odd and even mode differential transmission lines within the same package.
It is an object of the invention to provide a semiconductor package wherein the impedance level of the conductors is established during the assembly of the circuit.
It is an object of the invention to provide a semiconductor package which is usable for many chip designs, and thus reduces the costs associated with tooling, inventory, and package design.
It is further an object that the interconnection of the conductors is compatible with either wire bond or flip chip interconnection.
Yet another objective is to provide a flexible conductor system which is usable with different types of packages or substrates.
It is an object of one embodiment of the invention to provide a conductor system wherein single ended and differential mode transmission can be mixed within the same conductor system.
The invention is a microelectronic device, and a method of fabricating the device, wherein the impedance of single ended or differential signal transmission lines is determined by the choice of which conductors are connected to ground during the assembly of the semiconductor device. Moreover, the device is applicable to pairs of conductors having odd or even mode impedance within the same conductor system.
In one embodiment, the device is a semiconductor package having a plurality of conductors arrayed in two (or more) parallel planes, and a ground conductor. Each of the primary conductors has the same dimensions and spacing between conductors, and each of the auxiliary conductors has the same dimensions and spacing. Conductors in the auxiliary or second plane substantially overlay the primary signal conductors in the first plane, and the impedance of any lead or lead pair is arbitrarily set by connecting the auxiliary conductors to ground or by leaving them floating. Differential pairs of signal conductors, either odd or even mode are set by connecting the auxiliary conductors to a ground contact.
In an alternate embodiment, a conductor system is provided wherein single ended and differential impedance modes are mixed within the same package or substrate. Conductors are arrayed in two (or more) parallel planes, with an available ground conductor, and the auxiliary conductors substantially overlay the primary conductors. Conductor dimensions and spacing are set to provide alternating impedance modes.